1. Field of the Invention
The present invention relates to an active hybrid circuit for use in a full duplex channel. In particular, this invention provides an active hybrid circuit to generate a duplicated voltage of a current output stage for reducing the energy of the transmitter echoing to the receiver when the hybrid circuit is operated in a full duplex channel.
2. Description of the Related Art
Due to the demand for information communication, such as the transmission of data, sound, and video, the data transmission rate has improved continually as the hardware and communication protocol have progressed to achieve high-speed and real time transmission. Information transmission and business transactions over the Internet have increased as personal computers, workstations and servers become more and more popular. For Ethernet, the transmission rate has also improved from the early original 10 Mbps of the IEEE 802.3 10BASE-T standard, 100 Mbps of the IEEE 802.3u 100BASE-T standard, to 1 Gbps of the IEEE 802.3ab 1000BASE-T standard. 1 Gbps Ethernet is also called a Gigabit Ethernet.
However, when a Gigabit Ethernet is operated over a full duplex channel, meaning a high-speed bi-directional transmission, the energy of the transmitter is coupled to the receiver of the transceiver. The signal at the receiver will be disturbed by the signal at the transmitter. If the signals are not processed appropriately, the signals at the receiver will be incorrect or the range of the transmitted signal will be shorter.
To solve the problems identified above, many hybrid circuits have been developed. Hybrid circuits can be classified into voltage output stages and current output stages. The voltage output stage includes transformer hybrids and resistive hybrids. The current output stage includes, for example, transformer hybrids, resistive hybrids, duplicated current hybrids and duplicated voltage generator hybrids.
U.S. Pat. No. 6,775,529, “Active resistive summer for a transformer hybrid”, discloses an active resistive summer, as shown in FIG. 1. The active resistive summer comprises an operational amplifier 11, a feedback resistor RF1 and a plurality of resistors R11, R12, R13. A composite signal and a replica signal are inputted into the active resistive summer and a receiving signal is produced. The replica signal is subtracted from the composite signal to obtain the receiving signal. This method needs to generate a duplicated current which passes through a high-precision resistor to obtain a duplicated voltage. Therefore, a trimming operation is necessary when the IC is packaged in order to obtain the high-precision resistor in the IC.
U.S. Pat. No. 6,744,831, titled as “Adaptive electronic transmission signal cancellation apparatus for full duplex communication”, discloses an adaptive electronic transmission signal cancellation apparatus. This apparatus generates a first duplicated transmission signal to cancel the signal in the receiver that comes from the transmitter. At the same time, by utilizing the characteristic of the transmitter and the first duplicated transmission signal, a second duplicated transmission signal is generated to cancel the excess current of the signal at the transmitter. In this apparatus, a trimming operation is also necessary when the IC is packaged.
U.S. Pat. No. 6,665,347, “Output driver for high speed Ethernet transceiver”, discloses an output driver combination, as shown in FIG. 2. The output driver combination comprises a current driver 20, a voltage driver 21, a transformer 22 and a plurality of resistors R23, R24, R25, R26, R27, R28, R29. This driver can operate in two operation modes, including high speed and low speed. However, the output driver cannot be used at high-speed and a low supply voltage.
U.S. Pat. No. 6,792,105, “Current-mode differential active hybrid circuit”, discloses a current-mode differential active hybrid circuit. The active hybrid circuit comprises a first output H-bridge (including a first transistor and a first output resistor), a drive circuit, a second output H-bridge and a second output resistor. The drawback of this device is its high pin count in the IC. Because the characteristics of the transformer are not perfect, the above method cannot completely eliminate the disturbance at the receiver caused by the transmitted signal.